This piece in the Times about the increase in options for people who want to take public transit to and from US airports hits on a really important point:

Kenneth Lin, a senior planning manager in New York for Parsons Brinckerhoff, a consulting company that advised the Port Authority on construction of the AirTrain JFK, says he is a devotee of public transit, including AirTrain JFK.

“It’s cheaper than a taxi and more reliable during rush hour,” he said. “It reduces stress, as it is less idiosyncratic than taxis, and it usually arrives on time. In a car or taxi, it could be a very fast or very slow trip, depending upon traffic.”

My emphasis.  Right there is a huge adavtange of elevated trains and subways.  Not only are they faster than traffic, they’re also better able to stick to a reliable schedule.  That eliminates a lot of uncertainty when you’re making your travel plans.  You never need to say, “Oh, it’ll take between 20 and 40 minutes to get there, depending on traffic.”  You just look at the schedule, make your plans, and go.